mtd: rawnand: gpmi: Use dma_request_chan() instead dma_request_slave_channel()

dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

Use using dma_request_chan() directly to return the real error code.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200227123749.24064-2-peter.ujfalusi@ti.com

drivers/mtd/nand/raw/gpmi-nand/gpmi-nand.c

@@ -1148,20 +1148,21 @@ static int acquire_dma_channels(struct gpmi_nand_data *this)
 {
 	struct platform_device *pdev = this->pdev;
 	struct dma_chan *dma_chan;
+	int ret = 0;
 
 	/* request dma channel */
-	dma_chan = dma_request_slave_channel(&pdev->dev, "rx-tx");
-	if (!dma_chan) {
-		dev_err(this->dev, "Failed to request DMA channel.\n");
-		goto acquire_err;
+	dma_chan = dma_request_chan(&pdev->dev, "rx-tx");
+	if (IS_ERR(dma_chan)) {
+		ret = PTR_ERR(dma_chan);
+		if (ret != -EPROBE_DEFER)
+			dev_err(this->dev, "DMA channel request failed: %d\n",
+				ret);
+		release_dma_channels(this);
+	} else {
+		this->dma_chans[0] = dma_chan;
 	}
 
-	this->dma_chans[0] = dma_chan;
-	return 0;
-
-acquire_err:
-	release_dma_channels(this);
-	return -EINVAL;
+	return ret;
 }
 
 static int gpmi_get_clks(struct gpmi_nand_data *this)